1. Field of the Invention
The present invention relates to a method and an apparatus to reduce NOx emissions in the exhaust gas from installations that form NOx as a result of combustion processes, in which the NOx decomposition occurs in the temperature range from approximately 450.degree. C. to approximately 850.degree. C. and in an oxidizing atmosphere with a fuel-air ratio (Lambda) is greater than approximately 1.
2. Background Information
Nitrogen oxides, NO.sub.x, including at least nitrogen monoxide, NO, and nitrogen dioxide, NO.sub.2, are formed in essentially all industrial combustion processes from atmospheric oxygen, atmospheric nitrogen and the compounds containing nitrogen in the fuel. At combustion temperatures above about 1600.degree. C., with sufficiently long dwell times of the combustion gases in the flame, the essential product formed from the molecular nitrogen can be nitrogen monoxide, NO.
The nitrogen monoxide NO formed above about 1600.degree. C. is a strongly endothermic compound and is partly decomposed into nitrogen and oxygen in the temperature range from approximately 800.degree. C. to about 450.degree. C. When rapidly cooled to temperatures below about 450.degree. C., nitrogen monoxide NO can be obtained in metastable form.
One example of such an industrial process is the production of cement clinker in production facilities with rotary kilns and suspension-type gas heat exchangers.
Cement clinker refers to the glass-like stone-like material of, for example, clay and limestone, fused in the cement formation process.
Cement clinker is generally produced by the thermal treatment of a prepared, preheated mixture of raw materials in a rotary kiln. In facilities that recover and recycle the kiln waste heat, the raw material thereby can pass through a number of gas heat exchanger stages, where it can be some or all of pre-heated, subjected to a preliminary neutralization or calcined, sintered into cement clinker in the sintering zone of the rotary kiln and then cooled.
Flame temperatures of approximately 2000.degree. C. can be reached in the sintering zone of the rotary kiln. In addition to nitrogen monoxide, NO, nitrogen dioxide, NO.sub.2, can also be formed, namely in a ratio of about 90% NO.sub.x to about 10% NO.sub.2.
The heat exchanger stages can generally be realized in the form of cyclones, in which cyclone heat exchangers the raw material, which raw material is heated by the hot waste gases flowing in countercurrent out of the rotary kiln, is separated. The kiln exhaust gas can be at a temperature of approximately 1000.degree. C. as it enters the lowest stage of the heat exchanger. The exhaust gas can leave the uppermost stage of the heat exchanger at a temperature of approximately 250.degree. C. to 350.degree. C.
The heat exchangers can also possibly be concurrent or cocurrent flow heat exchangers.
The percentage of nitrogen oxides, NO.sub.x, in the exhaust gas as a whole can be different from plant to plant, because there can be different operating conditions in all furnaces. Within a single furnace, large fluctuations can also be observed in the concentrations of the nitrogen oxides, depending, at least in part, on the individual operating conditions and the different fuels used.
To comply with the limits specified by the applicable laws and regulations, measures are typically required to reduce the content of nitrogen oxides. In known devices, and taking into consideration the interrelationships of the fuel characteristics, flame temperature, flame type, hold time and fuel-air ratio, it is known that at least some parameters can be influenced, primarily to reduce the formation of nitrogen oxides. In the parts of the installation that are downstream in the direction of the exhaust gas, however, the dwell time of the kiln gases in the temperature range of approximately 450.degree. C. to approximately 800.degree. C., has not so far been taken into consideration as a factor that can be influenced regarding the NO.sub.x reduction.
In other words, the hold up time of kiln gases in the temperature range of about 450.degree. C. to about 800.degree. C., has essentially not been utilized as a variable that can substantially reduce the amount of NO.sub.x emissions.
The conditions in the heat exchanger are not static but a dynamic equilibrium. Therefore the NO.sub.x decomposition can be very decisively a function of the dwell time of the gas in the temperature range from approximately 450.degree. C. to approximately 800.degree. C. Accordingly, the decomposition rate can be determined by the distribution of the air flow velocity in the relevant temperature range of the heat exchanger. Thus the decomposition, as well as the formation, can be influenced by the temperature, the fuel-air ratio (Lambda) and by the dwell time.
For example, known publication ZKG International 49 (1996), No. 10, Pages 545 to 560, describes a method to reduce the NO.sub.x concentration in the exhaust gases from a rotary kiln by the addition of the reduction agent ammonia, NH.sub.3, in gas form or also in the form of ammonia liquor or ammonia water or ammonium hydroxide in the heat exchanger area. However, an optimum NH.sub.3 reduction is achieved only at higher temperatures of approximately 900.degree. C.
Known processes also teach that other process measures can be adopted, e.g. by an excess of carbon monoxide, CO, to achieve reducing conditions where Lambda is less than 1 in the calcining furnace, and to have a positive influence on the decomposition of nitrogen oxide, NO, by creating longer reaction times and retaining the reducing conditions. It must also be taken into consideration, however, that the allowable limits for CO emissions cannot be exceeded. For example, a graduated combustion technology is required for the secondary combustion of the CO.
The method of the prior art for use on combustion engines, as disclosed in German Patent No. 196 11 898 A1, operates on the basis of a fundamentally different method with oxygen enrichment and the extensive exclusion of atmospheric nitrogen from participation in the combustion process, so that a sharply reduced NO.sub.x formation occurs as a result of the nitrogen content of the fuel.
One disadvantage of these process control measures is that they can require high capital investments for the special construction of the plant and additional equipment. The conversion of existing plants can be particularly expensive. The use of NH.sub.3, in selective non-catalytic reduction, for example, can also result in high operating costs. Additional safety equipment can be required, for example.